1. Field of the Invention
The invention relates to a liquid crystal display which can display images in a wide range of environment from a light place to a dark place and execute a transmissive display with a wide viewing angle and high picture quality.
2. Description of the Related Art
At present, a transmissive liquid crystal display of an IPS (In Plane Switching) system, a VA (Vertical Alignment) system, or the like has been spread as a monitor and is also used as a television by improving response performance. A liquid crystal display has also been spread to the fields of portable information apparatuses such as cellular phone and digital camera. Although the portable information apparatus is mainly used personally, in recent years, the number of portable information apparatuses in which an inclination angle of a display unit can be varied has been increased and the display unit is often observed from the oblique direction. Therefore, a wide viewing angle is demanded.
Since the display for the portable information apparatus is used in a variety of environments in ranges from the outdoors in the fine weather to the darkroom, it is demanded that the display is transflective. In the transflective liquid crystal display, a reflective display unit and a transmissive display unit are arranged in one pixel.
In this case, the reflective display unit performs a display by reflecting a light entering from the ambience with use of a reflection layer and a contrast ratio is kept constant irrespective of the ambient brightness, so that a good display state can be obtained under a relatively light environment in ranges from the outdoors in the fine weather to the interior of the room. According to the transmissive display unit, since a backlight is used and the brightness is kept constant irrespective of the environment, a display of a high contrast ratio can be obtained in a relatively dark environment in ranges from the interior of the room to the darkroom. According to the transflective liquid crystal display having functions of both of them, a display of the high contrast ratio can be obtained in a wide range of environment from the outdoors in the fine weather to the darkroom.
Hitherto, it has been expected that the reflective display and the transmissive display of a wide viewing angle are simultaneously obtained by constructing the display of the IPS system known as a transmissive display of the wide viewing angle as a transflective type. The transflective IPS system has been disclosed in, for example, JP-A-11-242226.
In the liquid crystal display of the transflective IPS system, although a plurality of retardation films are arranged on the whole upper and lower surfaces of a liquid crystal panel, phase differences among the retardation films have viewing angle dependency. Therefore, even if the phase differences among the liquid crystal layer and the plurality of retardation films and an axis layout are optimized in a normal direction, as a viewing point gets away from the normal direction, conditions rapidly deteriorate to be away from optimum conditions for the dark display.
Although the viewing angle dependency of the phase differences can be reduced by adjusting a refractive index in the thickness direction of the retardation films, it cannot be completely eliminated. In the transflective IPS system, thus, an increase in dark display transmissive ratio in the oblique direction is large and viewing angle performance of the transmissive display is inferior to that of the transmissive IPS system.
According to JP-A-2003-279957, in the VA system, retardation films are arranged in close vicinity to the liquid crystal layer, patterned, and arranged only in the reflective display unit. However, nothing is disclosed with respect to application to the IPS system which provides the transmissive display with the wide viewing angle.
According to the transmissive IPS system, the liquid crystal layer has homogeneous alignment, upper and lower polarization films are arranged so that their transmissive axes cross perpendicularly, and one of the transmissive axes is parallel with the liquid crystal alignment direction. Since the light entering the liquid crystal layer is linearly polarized light and its electric vector is parallel with the liquid crystal alignment direction, the phase differences are not obtained by the liquid crystal layer. Therefore, since a dark display of a low transmissive ratio can be realized and no retardation films exist between the liquid crystal layer and the polarization films, a surplus phase difference does not occur in the oblique direction and the dark display with the wide viewing angle can be realized. As mentioned above, the retardation films are inherently unnecessary in the transmissive IPS system.
In the liquid crystal display of the transflective IPS system, the reflective display unit and the transmissive display unit in which optical conditions for the dark display are essentially different are arranged in one pixel. That is, in the reflective display unit, the light enters from the polarization film on the upper surface of the liquid crystal display, is reflected by the reflection layer in the liquid crystal panel, thereafter, passes through the upper polarization film again, and is directed toward the user. In the transmissive display unit, the light enters from the polarization film on the lower surface of the liquid crystal display, thereafter, passes through the polarization film on the upper surface of the liquid crystal display, and is directed toward the user. The phase difference between the phase of the light which provides the dark display in the reflective display unit and that in the transmissive display unit is caused due to such a difference between optical paths and it is equal to a quarter wave. Therefore, when the reflective display unit is in the light display mode, the transmissive display unit is in the dark display mode or vice versa, and the reflective display unit and the transmissive display unit have different applied voltage dependency. To allow those display units to have the same applied voltage dependency, the phase difference between the reflective display unit and the transmissive display unit has to be shifted by the quarter wave by some method.
According to the conventional transflective IPS system, the retardation films are arranged on the whole upper and lower surfaces of the liquid crystal panel. The light which enters the reflective display unit from the outside, the light reflected by the reflection layer of the reflective display unit, and the light which passed through the transmissive display unit pass through the retardation films on the upper side of the liquid crystal panel among those retardation films. As mentioned above, the upper retardation films act on both of the reflective display unit and the transmissive display unit. On the other hand, since only the light which is emitted from a light source and enters the transmissive display unit passes through the retardation films on the lower side of the liquid crystal panel, the lower retardation films act only on the transmissive display unit. By using a difference between the actions of the upper retardation films and the lower retardation films onto the reflective display unit and the transmissive display unit, the phase difference between both of the display units is shifted by the quarter wave. However, since the surplus phase difference occurs in the oblique direction since the retardation films exist between the liquid crystal layer and the polarization films, the viewing angle performance of the dark display deteriorates.